1. Field of the Invention
The present invention relates to a ceramics heater employed for a toner image heating and fixing device in a facsimile, a copying machine, a printer or the like.
2. Description of the Prior Art
In general, a toner image heating and fixing device in an image forming apparatus such as a facsimile, a copying machine or a printer transfers a toner image formed on a photoreceptor drum onto a transfer material and thereafter heats and pressurizes the transfer material while holding and transporting the same between a heating roller and a pressure roller, thereby fixing the unfixed toner image onto the transfer material. The conventional heating roller employed in the heating and fixing device is formed by setting a heat source such as a halogen lamp in a cylindrical metal roll for heating a surface part of the metal roll.
A toner image heating and fixing device employing a ceramics heater as a heating part thereof has been recently proposed and put into practice. The ceramics heater employed for such a device comprises a thin plate type electrical insulating ceramics substrate, a linear heat generator provided on a surface thereof and a protective layer of glass or the like covering a surface of the heat generator, and heat generator is energized for heating. A heating and fig device employing such a ceramics heater is described in Japanese Patent Laying-Open Nos. 1-263679 (1989), 2-157878 (1990), 63-313182 (1988) or the like, for example.
FIG. 1 shows an example of such a heating and fixing device. Referring to FIG. 1, a ceramics heater 1 of the aforementioned type is mounted on a support 2 of resin and a heat-resistant film 3 is rotatably provided on the outer peripheral portion of the support 2, while a pressure roller 4 is arranged to face the ceramics heater 1 through the heat-resistant film 3. A transfer material 5 having unfixed toner images 6a is held between the pressure roller 4 and the heat-resistant film 3 and carried or transported at a constant speed, so that toner images 6b are fixed onto the transfer material 5 due to pressurization by the pressure roller 4 and heating by the ceramics heater 1.
This heating and fixing device can reduce power consumption since the heat capacity of the ceramics heater is extremely smaller than that of the conventional metal roll, and is excellent in quick start performance since the heater requires no preheating upon switching on the power supply. The ceramics substrate forming the ceramics heater is generally prepared from alumina (Al.sub.2 O.sub.3).
In recent years, a higher fixing speed is required for the heating and fixing device employing the aforementioned ceramics heater. While the current ceramics heater employing an alumina substrate has a fixing speed of 4 to 8 ppm (pages per minute) for A4 (Japanese Industrial Standard) papers, a higher speed of at least 12 ppm is recently required.
In the ceramics heater, a voltage of 100 or 200 V is generally applied to one or each end of the heat generator to generate Joule heat of at least several 100 W, thereby increasing the temperature of the heater to about 200.degree. C. in about two to six seconds. When the fixing speed is increased, the time for transmitting the heat from the heater to each paper is reduced. However, a constant heating value is necessary for fixing the toner image and hence the heater must supply a larger quantity of heat per unit time, followed by application of a larger thermal shock to the heater.
In the ceramics heater employing an alumina substrate, however, a temperature difference arises between a portion around the heat generator and the remaining portion since alumina has a relatively small thermal conductivity of not more than 20 W/mK. On the other hand, such temperature difference results in thermal stress since alumina has a relatively large thermal expansion coefficient of 7.3 ppm/.degree. C. Therefore, the general alumina substrate is easy to crack when the temperature of the heater is increased. Thus, the alumina substrate is unsuitable for high- speed processing involving a large thermal shock.
To this end, a ceramics heater employing a substrate of aluminum nitride (AlN) in place of the alumina substrate having inferior thermal shock resistance has been recently developed, as described in Japanese Patent Laying-Open Nos. 9-80940 (1997) or 9-197861 (1997). According to Japanese Patent Laying-Open No. 9-80940, the temperature responsiveness of the heater is improved due to the high thermal conductivity of aluminum nitride. According to Japanese Patent Laying-Open No. 9-197861, on the other hand, improvement of fixability, capability of high-speed printing and reduction of power consumption are attained through the high thermal conductivity of aluminum nitride.
As hereinabove described, the conventional ceramics heater for a heating and fixing device employs a ceramics substrate of alumina or aluminum nitride. However, the ceramics heater employing an alumina substrate is unsuitable for improving the fixing speed since the substrate is readily cracked by a thermal shock. Whether the ceramics heater employs the alumina substrate or the aluminum nitride substrate, further, a defective connection is readily caused between the electrodes of the heat generator and a connector, to result in inferior connection reliability, especially correpondingly following a size increase of the transfer material.
The heating and fixing device is also required to fix a toner image onto a large-sized transfer material such as an A3 (Japanese Industrial Standard) paper, for example. However, the conventional heating and fixing device for fixing a toner image onto an A4 paper while vertically carrying the A4 (Japanese Industrial Standard) paper cannot fix the image onto an A3 paper. In order to attain fixation of the toner image onto the A3 paper, therefore, the length of the ceramics heater is increased.
In this case, the length of the heat generator provided on the ceramics substrate is remarkably increased from about 220 mm for the A4 paper to about 300 mm for the A3 paper, and the temperature of the heat generator reaches about 200 to 250.degree. C. Following heat generation of the heater, the alumina substrate is thermally expanded by 0.32 mm for the A4 paper or by 0.44 mm for the A3 paper when the heater temperature is 225.degree. C. and the room temperature is 20.degree. C., for example. The connector which is formed on the support for feeding the heat generator is generally prepared by plating a conductor mainly composed of copper having a low small resistance with a metal such as Ni for ensuring heat resistance.
When the ceramics substrate is expanded due to heat generation of the heater as hereinabove described, therefore, the metal such as Ni plated on the surface of the connector provided on the support readily comes off due to friction with the electrodes of the heat generator provided on the ceramics substrate, to expose the copper. The exposed copper is rapidly oxidized in the portions connected with the electrodes due to application of heat from the heater to form CuO having no conductivity, leading to defective connection between the connector and the electrodes of the heat generator.
The substrate of aluminum nitride having a smaller thermal expansion coefficient than alumina hardly causes the aforementioned problem of defective connection between the electrodes and the connector resulting from expansion of the substrate. However, the thermal conductivity of aluminum nitride is so high that heat generated in the heat generator is readily transmitted to the connector of a feeder part. Thus, the copper forming the connector is readily oxidized by the heat, to result in defective connection between the electrodes and the connector due to the oxidation.